Ban `--no-cache` with `--link-mode=symlink` (#5519)

## Summary

Closes https://github.com/astral-sh/uv/issues/5360.

crates/install-wheel-rs/src/linker.rs

@@ -258,6 +258,11 @@ impl LinkMode {
             Self::Symlink => symlink_wheel_files(site_packages, wheel, locks),
         }
     }
+
+    /// Returns `true` if the link mode is [`LinkMode::Symlink`].
+    pub fn is_symlink(&self) -> bool {
+        matches!(self, Self::Symlink)
+    }
 }
 
 /// Extract a wheel by cloning all of its files into site packages. The files will be cloned

crates/uv-cache/src/lib.rs

@@ -120,7 +120,7 @@ pub struct Cache {
     ///
     /// Included to ensure that the temporary directory exists for the length of the operation, but
     /// is dropped at the end as appropriate.
-    _temp_dir_drop: Option<Arc<tempfile::TempDir>>,
+    temp_dir: Option<Arc<tempfile::TempDir>>,
 }
 
 impl Cache {
@@ -129,7 +129,7 @@ impl Cache {
         Self {
             root: root.into(),
             refresh: Refresh::None(Timestamp::now()),
-            _temp_dir_drop: None,
+            temp_dir: None,
         }
     }
 
@@ -139,7 +139,7 @@ impl Cache {
         Ok(Self {
             root: temp_dir.path().to_path_buf(),
             refresh: Refresh::None(Timestamp::now()),
-            _temp_dir_drop: Some(Arc::new(temp_dir)),
+            temp_dir: Some(Arc::new(temp_dir)),
         })
     }
 
@@ -271,7 +271,12 @@ impl Cache {
         Ok(id)
     }
 
-    /// Initialize the cache.
+    /// Returns `true` if the [`Cache`] is temporary.
+    pub fn is_temporary(&self) -> bool {
+        self.temp_dir.is_some()
+    }
+
+    /// Initialize the [`Cache`].
     pub fn init(self) -> Result<Self, io::Error> {
         let root = &self.root;
 

crates/uv-dispatch/src/lib.rs

@@ -288,6 +288,7 @@ impl<'a> BuildContext for BuildDispatch<'a> {
             );
             wheels = Installer::new(venv)
                 .with_link_mode(self.link_mode)
+                .with_cache(self.cache)
                 .install(wheels)
                 .await
                 .context("Failed to install build dependencies")?;

crates/uv-installer/src/installer.rs

@@ -7,11 +7,13 @@ use tokio::sync::oneshot;
 use tracing::instrument;
 
 use distribution_types::CachedDist;
+use uv_cache::Cache;
 use uv_python::PythonEnvironment;
 
 pub struct Installer<'a> {
     venv: &'a PythonEnvironment,
     link_mode: LinkMode,
+    cache: Option<&'a Cache>,
     reporter: Option<Box<dyn Reporter>>,
     installer_name: Option<String>,
 }
@@ -22,6 +24,7 @@ impl<'a> Installer<'a> {
         Self {
             venv,
             link_mode: LinkMode::default(),
+            cache: None,
             reporter: None,
             installer_name: Some("uv".to_string()),
         }
@@ -33,6 +36,15 @@ impl<'a> Installer<'a> {
         Self { link_mode, ..self }
     }
 
+    /// Set the [`Cache`] to use for this installer.
+    #[must_use]
+    pub fn with_cache(self, cache: &'a Cache) -> Self {
+        Self {
+            cache: Some(cache),
+            ..self
+        }
+    }
+
     /// Set the [`Reporter`] to use for this installer.
     #[must_use]
     pub fn with_reporter(self, reporter: impl Reporter + 'static) -> Self {
@@ -54,16 +66,25 @@ impl<'a> Installer<'a> {
     /// Install a set of wheels into a Python virtual environment.
     #[instrument(skip_all, fields(num_wheels = %wheels.len()))]
     pub async fn install(self, wheels: Vec<CachedDist>) -> Result<Vec<CachedDist>> {
-        let (tx, rx) = oneshot::channel();
-
         let Self {
             venv,
+            cache,
             link_mode,
             reporter,
             installer_name,
         } = self;
-        let layout = venv.interpreter().layout();
 
+        if cache.is_some_and(Cache::is_temporary) {
+            if link_mode.is_symlink() {
+                return Err(anyhow::anyhow!(
+                    "Symlink-based installation is not supported with `--no-cache`. The created environment will be rendered unusable by the removal of the cache."
+                ));
+            }
+        }
+
+        let (tx, rx) = oneshot::channel();
+
+        let layout = venv.interpreter().layout();
         rayon::spawn(move || {
             let result = install(wheels, layout, installer_name, link_mode, reporter);
             tx.send(result).unwrap();
@@ -77,6 +98,14 @@ impl<'a> Installer<'a> {
     /// Install a set of wheels into a Python virtual environment synchronously.
     #[instrument(skip_all, fields(num_wheels = %wheels.len()))]
     pub fn install_blocking(self, wheels: Vec<CachedDist>) -> Result<Vec<CachedDist>> {
+        if self.cache.is_some_and(Cache::is_temporary) {
+            if self.link_mode.is_symlink() {
+                return Err(anyhow::anyhow!(
+                    "Symlink-based installation is not supported with `--no-cache`. The created environment will be rendered unusable by the removal of the cache."
+                ));
+            }
+        }
+
         install(
             wheels,
             self.venv.interpreter().layout(),

crates/uv/src/commands/pip/operations.rs

@@ -463,6 +463,7 @@ pub(crate) async fn install(
         let start = std::time::Instant::now();
         wheels = uv_installer::Installer::new(venv)
             .with_link_mode(link_mode)
+            .with_cache(cache)
             .with_reporter(InstallReporter::from(printer).with_length(wheels.len() as u64))
             // This technically can block the runtime, but we are on the main thread and
             // have no other running tasks at this point, so this lets us avoid spawning a blocking

crates/uv/tests/pip_sync.rs

@@ -236,6 +236,34 @@ fn install_symlink() -> Result<()> {
     Ok(())
 }
 
+/// Reject attempts to use symlink semantics with `--no-cache`.
+#[test]
+fn install_symlink_no_cache() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("MarkupSafe==2.1.3")?;
+
+    uv_snapshot!(context.pip_sync()
+        .arg("requirements.txt")
+        .arg("--link-mode")
+        .arg("symlink")
+        .arg("--no-cache")
+        .arg("--strict"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    Prepared 1 package in [TIME]
+    error: Symlink-based installation is not supported with `--no-cache`. The created environment will be rendered unusable by the removal of the cache.
+    "###
+    );
+
+    Ok(())
+}
+
 /// Install multiple packages into a virtual environment.
 #[test]
 fn install_many() -> Result<()> {